A deer mouse makes a nighttime raid to gather food in a hazelnut thicket.

December 2002

The midnight marauder

This "house guest" is up and about kind of late in winter.

John Yost

I wake to what sounds like a miniature drum, followed by the telltale cadence of running feet. Startled, I flick on the light hoping to catch a glimpse of the looter. But like every other time this scenario has played out, the rascal disappears into a hole in the wall before I can get a good look.

It's 4:30 a.m., the busiest time of day for a working woodland deer mouse (Peromyscus maniculatus). The little rodent is busy stuffing a large food cache with black oil sunflower seeds from my bird feeder. In this case the "cavern" she's filling happens to be the felt liner of my mukluks.

Every couple of weeks I check to see how much birdseed she has stored in her pantry. The supply of nuts grows during spring, summer and fall as she pilfers goodies from the feeders that I keep full year-round. When the snow flies, I reclaim my boots and lend her the use of my tennis shoes for the winter.

My furry companion and I share a cabin in the middle of a forest rich in oak, elm, spruce and balsam fir. This is perfect habitat for a woodland deer mouse. Abundant throughout Wisconsin, these tan rodents with white underparts inhabit deciduous, coniferous and mixed forests. They are abundant in manmade buildings, often residing in old sawmills, shacks and cabins. Another subspecies of deer mouse, the prairie deer mouse (P.m. bairdii), inhabits hay and grain fields and especially likes the fencerows where vines or shrubs provide additional protection.

It's in these fencerows where habitat varies between open field and shrub that anomalies of the mammalian world occur. The female prairie deer mice grow bigger than males and establish larger home ranges as well. Maintaining a bigger home range is helpful when raising her young, as a female needs two to three times her normal food intake to produce milk for offspring.

Deer mice give birth to one to eight young, and usually have three to five in a litter. The female can have four litters or more a year in a warm winter. The tiny pink babies are blind for the first two weeks of their lives. During this time they often remain firmly attached to their mother's teats for nourishment and protection. If startled, the female can run from the nest with her family in tow. Two to three weeks after their eyes open, the babies are weaned and ready to leave the nest.

Woodland deer mice prefer to nest in elevated cavities – often in woodpecker holes, birdhouses, or knotholes. In the case of my cabin mouse, it is happy to live in the insulation between the drywall and roof. I can always find its nest by the presence of Owens-Corning's finest fiberglass strewn about the floor below her new home.

Nests in the woods are almost as easily found. If you happen upon a 10- to 12-inch high pile of grasses, leaves and moss built on an old bird nest, you have undoubtedly found a deer mouse home. A pregnant female builds the spherical pile when the young are about to be born. She will use the nest for about a month while she rears her young, then move on to another site.

Males build nests throughout the year as well; changing sites as their abode becomes fouled with feces and urine, which all deer mice deposit in the nest. Male and female nests can be distinguished, if you have a little patience. Over the course of a week or two, listen for the tiny, almost imperceptible squeaks of the babies coming from the cavity. It's not uncommon for the adult mouse to sense your presence and silence the newborns, so you'll probably have to wait quietly for at least ten minutes before you'll hear anything. If you don't hear anything from the nest after two weeks of listening, you most likely have a male nest or an abandoned nest left behind for a less odiferous site.

Prairie deer mice prefer to reside on or slightly under the ground. You'll find their nests beneath a log or board, in an old tire, or inside an abandoned chipmunk burrow. The prairie deer mice also live among piles of rocks left along farm fields where the little critters hunt and store food for winter.

Deer mice are omnivorous, feeding on both plants and small bugs. In winter, deer mice feed on the insect larva, pupa and dormant adults as well as oak acorns, hickory nuts, beech nuts, pine cones and hazelnuts that are stockpiled in winter caches. These root cellars sometimes contain huge quantities of food, up to two gallons, although a pint or quart is more common.

The stored larder is gone by spring or summer when the rodents turn to a diet heavier in insects. They even tackle grasshoppers, which are large if you are the size of a deer mouse. The mouse at my cabin will sometimes sit outside the window at night waiting until a particularly delectable moth is attracted to the light.

A little later in the season, fruits including pin cherries, blueberries, raspberries, blackberries and partridgeberries are eaten. I even discovered a mouse-sized supply of gooseberries in a floor crevice when I was remodeling last summer.

While the deer mouse in my cabin is safe eating her gooseberries, mice living in less domestic haunts are eaten by a host of predators. Mammals like weasels, striped skunks, raccoons, foxes, coyotes and domestic cats prey heavily on the small cricetids. A variety of hawks and all owl species in Wisconsin feed on unwary mice. At the cabin, a great horned owl often perches in the spruce tree next to the mouse's favorite feeder.

So far the four-legged creature has safely gathered seed from the tray outside the window. Seemingly fearless, she isn't even bothered by my face pressed against the glass as I watch her antics. As if she knows there's a barrier between us, she darts out of a small gap in the sill, grabs a seed, and returns to the space between the walls. I suspect she piles the provisions there, waiting for me to go to sleep before filling my boot liners. It's only when I stir that she becomes alarmed and rapidly stamps her feet against the floor in a warning to all before vanishing into a crevice.

Watch those droppings

Though deer mice are cute to see, be careful when sweeping up their droppings. Deer mouse feces, urine and saliva can carry a variety of Hantaviruses that can cause a respiratory illness that has sickened and even killed people. Symptoms of Hantavirus exposure include fever, chills, occasional headaches and possibly digestive problems. A few days later, breathing problems can start including coughing and a shortness of breath. The actual number of people affected since this disease was first described is quite low, but take these precautions.

If you are cleaning out a cabin, garage or room, particularly one that has been closed up for some time, open the doors and windows to get plenty of fresh air flow before you start your cleaning. don't vacuum or sweep mouse droppings; which might suspend the infective organism. Wear rubber gloves, thoroughly wet the area with a disinfectant, and then carefully wipe up the droppings with a paper towel and dispose of them in the trash. If you are kicking up a lot of dust during your cleaning, please consider wearing a facemask or respirator as well.

Animals are not known to get sick from this virus and, other than the rodent species directly involved, no other animals are known to spread it. For example, a cat that eats an infected deer mouse will neither get sick nor will it spread the disease.